The concept of producing plied yarns using the false-twist, self-twist phenomenon are now rather well known in the art. Documents in which the general principles of false-twisting and self-twisting are described include the following:
"Self-Twist Yarn," D. E. Henshaw, Merrow Publishing Co., Ltd., Watford, Herts, England, 1971; U.S. Pat. Nos.: PA1 Re 27,717, breen et al; PA1 3,225,533 Henshaw; PA1 3,306,023, Henshaw et al; PA1 3,353,344, Clendening, Jr.; PA1 3,434,275, Backer et al; PA1 3,507,108, Yoshimura et al; PA1 3,717,988, Walls; PA1 3,775,955, Shah; PA1 3,940,917, Strachan.
For purposes of convenience, some general comments concerning producing plied yarn by these techniques will be described. It is possible to form a plied yarn by false-twisting two or more singles yarn strands, attaching the strands to each other and then permitting the strands to wrap about each other using the release of forces stored by the false-twisting to accomplishing the plying, hence the term "self-twist." The false-twisting itself, in simplified form, involves holding spaced points of a yarn strand and twisting the strand in one direction at a point intermediate the held points, e.g., the center. This produces twists on one side of the center in one direction and on the other side of the center in the opposite direction. The center of the twisted strand constitutes a point of twist reversal and is called a "node." Clearly, forces are stored in the strand in the twisting step. When two strands similarly false-twisted are brought together in side-by-side juxtaposition and permitted to act against or with each other by releasing the nodes, the stored forces cause the strands to ply, i.e., to wrap around each other spontaneously. The process is enhanced and the product made more stable if the nodes of the two strands are aligned and are joined or locked together before release and plying.
As will be recognized, the torque or twist force exerted by each strand is roughly proportional to the amount of twist therein and that such force decreases as the strands ply. The plying step itself therefore continues until the stored twist forces in each strand decrease to a point at which the remaining twist forces are exactly counterbalanced by the resistance to further twisting in the plied yarn. Thus, if one begins with individual strands and then false twists the strands and plies them, each strand will end up, in the plied yarn, with some degree of false-twist which can be thought of as some remaining stored potential energy, the force exerted thereby being too small to cause further ply twisting against opposing frictional forces in the plied yarn.
An apparatus for false-twisting and node fastening and plying singles yarn strands is fully described in U.S. Pat. Ser. No. 755,671, filed Dec. 30, 1976 now U.S. Pat. No. 4,074,511, wherein a rotatable guide member receives the false-twisted yarn and fastens the nodes thereof using a rotating contact device carried by the guide member.
While the apparatus is quite satisfactory, it has been found desirable to assist the self-twist plying process and also to add bulk to the yarn in a practical continuous process for producing plied yarn.